Display percentage progress for encode stages

[peskdale]

'Encoding Pass X' isn't really sufficient I feel - especially when dealing with longer videos which can take most of a day to encode.

This gist might be a good starting point: https://gist.github.com/marazmiki/3015621

[rrah]

Tom did try this, at the same time as doing the file move percentage. Can't remember how it was done, but it caused slow-down with the encoding as it would pause, report the percentage, then resume

[tjenkinson]

We use the ffmpeg -progress flag and then read the output file it generates every second and calculate progress from that.

Send program-friendly progress information to url.

Progress information is written approximately every second and at the end of the encoding process. It is made of "key=value" lines. key consists of only alphanumeric characters. The last key of a sequence of progress information is always "progress".

I decided to go for this over parsing the stdout as I couldn't find the structure of the stdout documented anywhere so was worried it might change in the future.

https://github.com/LA1TV/Website-Upload-Processor/blob/master/src/main/java/uk/co/la1tv/websiteUploadProcessor/helpers/FfmpegProgressMonitor.java

https://www.ffmpeg.org/ffmpeg.html#Main-options